Laser power monitoring in a heat-assisted magnetic recording device using a resistive sensor and high-frequency laser modulation

ABSTRACT

An apparatus comprises a light source configured to generate light, and a modulator coupled to the light source and configured to modulate the light above a predetermined frequency. A slider is configured for heat-assisted magnetic recording and to receive the modulated light. A resistive sensor is integral to the slider and subject to heating by absorption of electromagnetic radiation and conduction of heat. Measuring circuitry is coupled to the resistive sensor and configured to measure a response of the resistive sensor due to absorbed electromagnetic radiation and not from the heat conduction. The measuring circuitry may further be configured to determine output optical power of the light source using the measured resistive sensor response.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. patent application Ser. No.15/190,462, filed Jun. 23, 2016, which is incorporated herein byreference in its entirety.

SUMMARY

Embodiments are directed to a method comprising modulating lightgenerated by a light source situated in, at, or near a slider above apredetermined frequency, the slider comprising a resistive sensor. Themethod also comprises communicating the modulated light from the lightsource, through the slider, and to an intended focus location of theslider. In response to the modulated light, the resistive sensor isheated by absorption of electromagnetic radiation and conduction of heatfrom heat sources proximate to the resistive sensor. The method furthercomprises measuring a response of the resistive sensor due to theabsorbed electromagnetic radiation and not from the heat conduction. Themethod may also comprise determining output optical power of the lightsource using the measured resistive sensor response.

Other embodiments are directed to an apparatus comprising a light sourceconfigured to generate light, and a modulator coupled to the lightsource and configured to modulate the light above a predeterminedfrequency. A slider is configured for heat-assisted magnetic recordingand to receive the modulated light. A resistive sensor is integral tothe slider and subject to heating by absorption of electromagneticradiation and conduction of heat. Measuring circuitry is coupled to theresistive sensor and configured to measure a response of the resistivesensor due to absorbed electromagnetic radiation and not from the heatconduction. The measuring circuitry may further be configured todetermine output optical power of the light source using the measuredresistive sensor response.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a HAMR slider with which variousembodiments disclosed herein may be implemented;

FIG. 2 is a cross-sectional view of a HAMR slider with which variousembodiments disclosed herein may be implemented;

FIG. 3 shows responses of two resistive sensors situated in a region ofa HAMR slider where the resistive sensors were obstructed from modulatedlaser light;

FIG. 4 shows the responses of two resistive sensors situated in a regionof a HAMR slider where the resistive sensors received modulated laserlight;

FIGS. 5A-5D illustrate the frequency-dependent nature of a resistivesensor of a HAMR recording head in accordance with various embodiments;

FIG. 6 illustrates a method for measuring a response of a resistivesensor in a HAMR slider in accordance with various embodiments;

FIG. 7 is a block diagram of a system for laser power monitoring in aHAMR drive using a resistive sensor and high-frequency laser lightmodulation in accordance with various embodiments; and

FIG. 8 is a block diagram of a system for laser power monitoring in aHAMR drive using a resistive sensor and high-frequency laser lightmodulation in accordance with various embodiments.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

The present disclosure generally relates to heat-assisted magneticrecording (HAMR), also referred to as energy-assisted magnetic recording(EAMR), thermally-assisted magnetic recording (TAMR), andthermally-assisted recording (TAR). This technology uses a laser sourceand a near-field transducer (NFT) to heat a small spot on a magneticdisk during recording. The heat lowers magnetic coercivity at the spot,allowing a write transducer to change the orientation of a magneticdomain at the spot. Due to the relatively high coercivity of the mediumafter cooling, the data is less susceptible to superparamagnetic effectsthat can lead to data errors.

Embodiments of a HAMR head 100 are illustrated in FIGS. 1 and 2. Asshown, the head 100 (also referred to as a slider) includes a lightsource (e.g., a laser diode) 102 located proximate a trailing edgesurface 104 of the slider body 101. An optical wave (e.g., a laser beam)120 generated by the light source 102 is delivered to an NFT 112 via anoptical waveguide 110. The NFT 112 is aligned with a plane of an airbearing surface (ABS) 114 of the head 100, and one edge of a read/writehead 113 is on the ABS 114. The read/write head 113 includes at leastone writer and at least one reader. In some embodiments, multiplewriters (e.g., 2 writers) and multiple readers (e.g., 3 readers) can beincorporated into the read/write head 113. The ABS 114 faces, and isheld proximate to, a surface 116 of a magnetic medium 118 during deviceoperation. The ABS 114 is also referred to as a media-facing surface.

The light source 102 in this representative example may be an integral,edge firing device, although it will be appreciated that any source ofelectromagnetic energy may be used. For example, a surface emittinglaser (SEL), instead of an edge firing laser, may be used as the source102. A light source may also be mounted alternatively to other surfacesof the head 100, such as the trailing edge surface 104. While therepresentative embodiments of FIGS. 1 and 2 show the waveguide 110integrated with the head 100, any type of light delivery configurationmay be used. As shown in FIG. 1, the laser diode 102 is shown coupled tothe slider body 101 via a submount 108. The submount 108 can be used toorient and affix an edge-emitting laser diode 102 so that its output isdirected downwards (negative y-direction in the figure). An inputsurface of the slider body 101 may include a grating, an opticalcoupler, or other coupling features to receive light from the laserdiode 102.

When writing with a HAMR device, electromagnetic energy is concentratedonto a small hotspot 119 over the track of the magnetic medium 118 wherewriting takes place, as is shown in the embodiment of FIG. 2. The lightfrom the light source 102 propagates to the NFT 112, e.g., eitherdirectly from the light source 102 or through a mode converter or by wayof a focusing element. FIG. 2, for example, shows an optical coupler 107adjacent the light source 102, which is configured to couple lightproduced from the light source 102 to the waveguide 110.

As a result of what is known as the diffraction limit, opticalcomponents cannot be used to focus light to a dimension that is lessthan about half the wavelength of the light. The lasers used in someHAMR designs produce light with wavelengths on the order of 700-1550 nm,yet the desired hot spot 119 is on the order of 50 nm or less. Thus, thedesired hot spot size is well below half the wavelength of the light.Optical focusers cannot be used to obtain the desired hot spot size,being diffraction limited at this scale. As a result, the NFT 112 isemployed to create a hotspot on the media.

The NFT 112 is a near-field optics device configured to generate localsurface plasmon resonance at a designated (e.g., design) wavelength. TheNFT 112 is generally formed from a thin film of plasmonic material on asubstrate. In a HAMR head 100, the NFT 112 is positioned proximate thewrite pole 226 of the read/write head 113. The NFT 112 is aligned withthe plane of the ABS 114 parallel to the surface 116 of the magneticmedium 118. The waveguide 110 and optional mode converter 107 and/orother optical element directs electromagnetic energy 120 (e.g., laserlight) onto the NFT 112. The NFT 112 achieves surface plasmon resonancein response to the incident electromagnetic energy 120. The plasmonsgenerated by this resonance are emitted from the NFT 112 towards themagnetic medium 118 where they are absorbed to create a hotspot 119. Atresonance, a high electric field surrounds the NFT 112 due to thecollective oscillations of electrons at the metal surface (e.g.,substrate) of the magnetic medium 118. At least a portion of theelectric field surrounding the NFT 112 gets absorbed by the magneticmedium 118, thereby raising the temperature of a spot 119 on the medium118 as data is being recorded.

FIG. 2 shows a detailed partial cross-sectional view of an embodiment ofthe HAMR head 100 in accordance with various embodiments. The waveguide110 includes a layer of core material 210 surrounded by first and secondcladding layers 220 and 230. The first cladding layer 220 is shownproximate the NFT 112 and the write pole 226. The second cladding layer230 is spaced away from the first cladding layer 220 and separatedtherefrom by the waveguide core 210. The core layer 210 and claddinglayers 220 and 230 may be fabricated from dielectric materials, such asoptical grade amorphous material with low thermal conductivities. Thefirst and second cladding layers 220 and 230 may each be made of thesame or a different material. The materials are selected so that therefractive index of the core layer 210 is higher than refractive indicesof the cladding layers 220 and 230. This arrangement of materialsfacilitates efficient propagation of light through the waveguide core210. Optical focusing elements (not shown) such as mirrors, lenses,etc., may be utilized to concentrate light onto the NFT 112. These andother components may be built on a common substrate using wafermanufacturing techniques known in the art. The waveguide 110 may beconfigured as a planar waveguide or channel waveguide.

According to some embodiments, the head 100 includes one or more contactsensors, such as the contact sensor 201 shown in FIG. 2. The contactsensor 201 can be configured to sense for one or more of head-mediumcontact, thermal asperities, and voids of a magnetic recording medium.The contact sensor 201 is preferably a resistive sensor that can beimplemented as a thermal sensor, such as a resistive temperature sensor(e.g., TCR sensor). For example, the contact sensor 201 can beimplemented as a DETCR (Dual Ended Thermal Coefficient of Resistance)sensor. The contact sensor 201 can be situated at or near the ABS 114and proximate the NFT 112. As such, the contact sensor 201 can serve asa temperature sensor for the NFT 112 and as a head-medium/asperitycontact sensor. In addition, the contact sensor 201 can serve as a laserpower monitor responsive to high-frequency laser light modulation inaccordance with various embodiments disclosed herein.

According to some embodiments, the head 100 shown in FIG. 2 canincorporate a bolometer 202 situated in the vicinity of the light path(e.g., the waveguide 110) and proximal of the NFT 112 in accordance withvarious embodiments. According to some embodiments, the bolometer 202comprises an optical-to-thermal transducer configured to respond tofluctuations in output optical power of the laser 102. In someembodiments, the bolometer 202 comprises a thin metallic wire placedacross the light path (e.g., the waveguide 110) within the internal bodyof the slider 100 at a location between the coupler 107 and the NFT 112.A small fraction of the output optical power of the laser 102transmitted via the light path is absorbed by the wire and convertedinto thermal power, thereby increasing wire temperature. Fluctuations inoutput optical power of the laser 102 correlate to fluctuations inthermal power and temperature of the bolometer 202. These fluctuationsin temperature can be detected by circuitry configured to monitor theresistance fluctuations in the wire by using a small bias current and ahigh thermal coefficient of resistance material. The bolometer 202 canbe placed in the light path or adjacent to optics to harvest scatteredlight. The bolometer 202 can serve as a laser power monitor responsiveto high-frequency laser light modulation in accordance with variousembodiments disclosed herein.

As shown in FIG. 2, the bolometer 202 has a longitudinal axis that isoriented transverse to the longitudinal axis of the waveguide 110. Moreparticularly, the bolometer 202 shown in FIG. 2 has a longitudinal axisthat is oriented substantially normal to the longitudinal axis of thewaveguide 110. In some embodiments, the bolometer 202 is spaced awayfrom a core 210 of the waveguide 110 and positioned above the waveguide110 in the x-direction. In other embodiments, the bolometer 202 isspaced away from a core 210 and positioned below the waveguide 110 inthe x-direction. Rather than being oriented normal to the waveguide 110,the longitudinal axis of the bolometer 202 can be oriented diagonallywith respect to the longitudinal axis of the waveguide 110. Orientingthe bolometer 202 diagonally with respect to the waveguide 110 serves toexpose more surface area of the bolometer 202 to optical energytransmitted along the waveguide 110 than a perpendicular orientation.

The output of a laser diode used in a HAMR drive is temperaturesensitive and susceptible to self-heating. During write operation, forexample, laser diode heating can vary the junction temperature of thelaser diode, causing a shift in laser emission wavelength, leading to achange of optical feedback from the optical path in the slider to thecavity of the laser diode, a phenomenon that is known to lead to modehopping and/or power instability of the laser diode. Mode hopping isparticularly problematic in the context of single-frequency lasers.Under some external influences, a single-frequency laser may operate onone resonator mode (e.g., produce energy with a first wavelength) forsome time, but then suddenly switch to another mode (produce energy,often with different magnitude, with a second wavelength) performing“mode hopping.” Temperature variation is known to cause mode hopping inlaser diodes. Mode hopping is problematic for HAMR applications, as modehopping leads to laser output power jumping and magnetic transitionshifting from one block of data to another. Large transition shifts in ablock of data may not be recoverable by channel decoding, resulting inerror bits. Accurate laser power monitoring can be particularly helpfulis avoiding mode hopping in HAMR devices.

Embodiments of the disclosure are directed to apparatuses and methodsfor measuring a response of a resistive sensor of a HAMR recording headdue to electromagnetic radiation absorption, and not to heat conduction.Embodiments of the disclosure are directed to apparatuses and methodsfor monitoring laser output optical power using a response of aresistive sensor of a HAMR recording head to electromagnetic radiationabsorption, and not from heat conduction. Embodiments of the disclosureare directed to apparatuses and methods for monitoring the lightdelivery system of a HAMR drive inside (in-situ) the drive.

According to various embodiments, a HAMR recording head (slider) isfabricated to incorporate a resistive sensor (e.g., a thermal sensor),such as a thin metal wire with nanometer cross-section dimensions, fordetecting head-disk contact and thermal asperities. One such resistivesensor is referred to herein as a Dual-Ended Thermal Coefficient ofResistance (DETCR) sensor. In addition to serving as a contact sensor, aDETCR can be used to detect and/or measure light in the HAMR slider.When laser light propagates through the slider in the optical waveguide,structures at various locations within the slider absorb light and aresubject to heating. For example, the writer pole is heated by lightabsorption and also produces heat. The DETCR may be heated by heatconduction from the writer pole and other heat sources of the slider inproximity to the DETCR. In addition, the DETCR may be heated by directlight absorption, referred to herein as absorption of electromagneticradiation.

The location of a DETCR in the slider and the characteristics of theDETCR make it a useful sensor for measuring the power of incident lightproduced by the light source (e.g., laser diode) of the slider. An idealsensor for measuring the power of incident light (e.g. an ideal laserpower monitoring sensor) is one that responds only to direct lightabsorption (absorption of electromagnetic radiation). However, as isdiscussed above, the EM radiation absorption response of the DETCR isconfounded by heating due to conduction of heat from surroundingstructures of the slider. In order to use the DETCR as a bolometer (asensor for measuring the power of incident light) with goodsignal-to-noise characteristics, it is desirable to separate the DETCRheating due to direct light absorption from heating due to heatconduction. Embodiments of the disclosure are directed to apparatusesand methods that utilize modulated light generated by a light source(e.g., laser diode) having frequencies at which only direct lightabsorption heats the DETCR.

The penetration depth at which time-varying heating occurs by thermalconduction within a HAMR slider is determined by the thermal diffusivityof the material being heated. With the complex structures in a HAMRslider, the thermal diffusivity is non-homogenous and therefore complexto calculate, but the measured thermal time constant is on the order ofseveral microseconds. In contrast, DETCR heating by direct lightabsorption is limited only by the heat capacity and thermal mass of theDETCR wire and can occur at speeds much faster than 1 microsecond. Bymeasuring the heating response of the DETCR as a function of lasermodulation frequency, the minimum response time of heat conduction to atime-varying signal can be identified. Once this is identified,measurements done with smaller response times, or higher frequencies,will only observe direct light absorption. The disclosed techniques canbe used for design validation, internal test methods, volume productiontesting, drive diagnostic testing, failure mode evaluation, in situmonitoring of light delivery and thermal changes, or for drive lifemonitoring, for example.

FIG. 3 shows the response of two DETCRs situated in a region of a HAMRslider in which the DETCR devices are obstructed from the waveguide bylarge light absorbing structures (writer pole and shields). That is, theDETCRs are not heated by direct light absorption. In this type ofdesign, the DETCRs are heated only by heat conduction from otherstructures within the slider. The y-axis of FIG. 3 is DETCR modulationamplitude, and the x-axis is laser modulation frequency (Hz). Curve 302is the response of a first DETCR, and curve 304 is the response of asecond DETCR. Both curves 302 and 304 show that the amplitude of theDETCR signals (indicative of sensor resistance) decreases withincreasing laser modulation frequency. FIG. 3 demonstrates that athigher laser modulation frequencies, heat is not conducted fast enoughand as a result, the DETCR signal continues to decrease to noise levels.

In FIG. 4, the light from a laser diode in a HAMR slider was modulatedby a small amount and the frequency at which the light was modulated wassystematically varied. In this illustrative embodiment, the modulationwas achieved by using a test feature of the drive preamp and standardlaboratory electronics. In this illustrative embodiment, the recordingheads were in full read-write operation on a spinstand tester. It isunderstood that this embodiment can be directly applied to volumetesting or drive operation (e.g., in situ a HAMR drive). The response totwo DETCRs (e.g., curves 402 and 404) was measured using a standardDETCR design and standard DETCR signal collection and amplificationelectronics. In contrast to FIG. 3, the two DETCRs were situated in aregion of a HAMR slider in which the DETCR devices were exposed to lightfrom the waveguide. In this case, the narrow-band power of the DETCRsignals was measured at each modulation frequency using a techniquesimilar to lock-in amplification. An alternative approach would be toperform Fourier transform analysis at the laser modulation frequency.

FIG. 4 shows the DETCR responses (curves 402 and 404) with two distinctfrequency regimes: one in which the narrow-band power decreases withincreasing frequency (low-frequency regime) and one in which thenarrow-band power is independent of frequency (high-frequency regime).For this particular resistive sensor design, this transition occurs near500-600 kHz. It is understood that the transition frequency, f_(T), willvary depending on the design of the DETCR or resistive sensor beingused, such as between 400 KHz and 1 MHz, for example. Other usefulranges include between about 200 kHz and 2 MHz, and between about 100kHz and 10 MHz.

It can be seen in FIG. 4 that the amplitude of the DETCR responses issubstantially independent of laser modulation frequency at frequenciesabove the transition frequency, f_(T), (see encircled data points 406).At frequencies above the transition frequency, f_(T), the DETCRresponses are indicative of direct light heating (due only to absorptionof electromagnetic radiation) and can be used to identify changes in theamount of light reaching the transducer region of a HAMR recording head.It can be seen in FIG. 3 that there is no high-frequency response regionfor the optically shielded DETCRs. A comparison between FIGS. 3 and 4confirms that the DETCR responses at frequencies above the transitionfrequency, f_(T), are purely due to direct light absorption and not fromthermal conduction. As such, a DETCR or other resistive sensor operatingin the high-frequency regime produces a response that is purely due toabsorption of electromagnetic radiation, and not from thermalconduction, which can be accurately correlated to output optical powerof the light source (e.g., laser diode) of the slider.

FIGS. 5A-5D illustrate the frequency-dependent nature of a resistivesensor, such as a DETCR sensor, in accordance with various embodiments.In FIG. 5A, a laser input signal is shown having a frequency thatincreases with time. The modulating laser input signal shown in FIG. 5Acan be used to produce a modulated laser output signal which iscommunicated through the slider to an intended focus location (e.g., anear-field transducer). In this illustrative example, it is assumed thata resistive sensor of the slider is either in the light path or receivesstray light that is exiting the optical waveguide or the slider. Theresistive sensor can be a DETCR, a bolometer, or other thermal sensor.

FIGS. 5B-5D are signals indicative of changes in resistance of theresistive sensor in response to modulated laser light generated by alaser diode driven with the laser input signal shown in FIG. 5A. FIG. 5Bshows the contribution of conductive heating to changes in theresistance of the resistive sensor. As can be seen in FIG. 5B, theamplitude of the conductive heating component of the resistive sensorresistance falls off exponentially with increasing laser input signalfrequency until a transition frequency, f_(T), is reached. At modulatedlaser light frequencies greater than the transition frequency, f_(T),the amplitude of the conductive heating component of the resistivesensor resistance is at or near zero.

FIG. 5B clearly illustrates that the resistive sensor has a response toconductive heating that falls into two distinct regimes, a low-frequencyregime (below f_(T)) and a high-frequency regime (above f_(T)). In otherwords, the AC component of the resistive sensor resistance due toconductive heating decreases with increasing modulated laser lightfrequency until a transition frequency, f_(T), is reached, after whichthe conductive heating component of the resistive sensor resistance isat or near DC (e.g., substantially devoid of the conductive heatingcomponent). FIG. 5B shows that the contribution from conductive heatingto changes in the resistance of the resistive sensor becomes negligibleat (and above) the transition frequency, f_(T).

FIG. 5C shows the contribution from electromagnetic radiation absorptionto changes in the resistance of the resistive sensor. It can be seen inFIG. 5C that the amplitude of the EM radiation absorption component ofthe sensor resistance is relatively constant for modulated laser lightfrequencies below and above the transition frequency, f_(T). In otherwords, the amplitude of the EM radiation absorption heating component ofthe sensor resistance does not vary with increasing modulated laserlight frequency.

FIG. 5D illustrates changes in the total resistance of the resistivesensor due to the combined contribution of heat conduction andelectromagnetic radiation absorption. In the low-frequency regime (belowf_(T)), the resistance of the resistive sensor changes due to both heatconduction and EM radiation absorption. In the high-frequency regime(above f_(T)), however, changes in the resistance of the resistivesensor is due only to EM radiation absorption, and not from thermalconduction.

Turning now to FIG. 6, a method is described for measuring the responseof a resistive sensor in a HAMR slider in accordance with variousembodiments. The method in FIG. 6 involves modulating 602 a frequency oflight generated by a light source above a predetermined frequency (e.g.,the transition frequency, f_(T)). The light source may be a laser diode,for example, situated in, at, or near a HAMR slider. The method involvescommunicating 604 the modulated light through the HAMR slider to anintended focus location of the slider (e.g., to a near-fieldtransducer). In response to the modulated light, a resistive sensor ofthe slider is heated 606 by absorption of electromagnetic radiation andconduction of heat from heat sources proximate the resistive sensor. Themethod further involves measuring 608 a response of the resistive sensordue to absorbed electromagnetic radiation and not from conducted heat.

The method may also involve determining 610 output optical power of thelight source using the measured resistive sensor response. In someembodiments, the resistive sensor can be a bolometer. In otherembodiments, the resistive sensor can be a contact sensor, such as DETCRsensor, which can serve as both a contact sensor (e.g., for head-diskcontact detection and/or thermal asperity detection) and an outputoptical power sensor for the light source (e.g., laser diode) of theslider.

FIG. 7 is a block diagram of a system for laser power monitoring in aHAMR drive using a resistive sensor and high-frequency laser lightmodulation in accordance with various embodiments. The system shown inFIG. 7 includes a light source 702, such as a laser diode, coupled to apower supply 704. The light source 702 is coupled to a modulator 706which is configured to modulate output light produced by the lightsource 702. The modulator 706 is configured to modulate light producedby the light source 702 above a predetermined frequency, such as thethreshold frequency, f_(T), discussed previously.

Modulated light produced by the light source 702 is communicated to aHAMR slider 710 which includes a resistive sensor 712. The modulatedlight has a frequency at which the response of the resistive sensor 712is due only to absorbed electromagnetic radiation and not from heatconduction. As was discussed previously, the modulation frequency of thelight is dependent on the transition frequency, f_(T), for theparticular design of the resistive sensor 712. As such, the modulatedlight has a frequency equal to or greater than the transition frequency,f_(T). The resistive sensor 712 can be a DETCR sensor, a bolometer, orother thermal sensor, for example. The resistive sensor 712 ispositioned in the light path of the HAMR slider 710 or other location ofthe slider 710 where stray light can be observed.

Measuring circuitry 714 is coupled to the resistive sensor 712. Themeasuring circuitry 714 is configured to receive a modulating (AC)resistance signal produced by the resistive sensor 712, such as theresistive sensor signal shown in FIG. 5D. The measuring circuitry 714 isconfigured to measure the response of the resistive sensor 712 as afunction of the modulation frequency of the light. In some embodiments,the measuring circuitry 714 is configured to measure the response of theresistive sensor at or near the predetermined frequency or at twice thepredetermined frequency of the modulated light. The measuring circuitry714, for example, can be configured to measure an average amplitude ofthe resistive sensor response. The measuring circuitry 714 produces anoutput signal 716 which can be stored in a memory 718.

In some embodiments, the measuring circuitry 714 is further configuredto determine output optical power of the light source 702 using themeasured resistive sensor response. The output signal from resistivesensor 712 can be used to compute the optical power of the light sourceonce the output has been calibrated. One method would be to calibratethe resistive sensor output to the light source input power while thedrive is executing factory calibration routines. The output signal 716,which is indicative of optical output power of the light source 702, canbe fed back to the power supply 704 and used to adjust the currentsupplied to the light source 702.

As was discussed previously, the resistive sensor 712 can be implementedas a contact sensor, such as a DETCR sensor. In such embodiments, theoutput from the resistive sensor 712 can be communicated to a detector720. The detector 720 is configured to detect contact and/or spacingchanges between the HAMR slider 710 and a magnetic recording mediumusing known techniques.

FIG. 8 is a block diagram of a system for laser power monitoring in aHAMR drive using a resistive sensor and high-frequency laser lightmodulation in accordance with various embodiments. FIG. 8 shows theelectrical architecture for performing modulated laser powermeasurements in accordance with various embodiments. The system shown inFIG. 8 includes a hard disk drive (HDD) controller 802 (e.g., an ASIC),a preamplifier 830, and a HAMR recording head 850. The HAMR recordinghead 850 includes a laser diode 852, a resistive sensor 854 (e.g., aDETCR or bolometer), and a write head 856. Although not shown, the HAMRrecording head 850 includes one or more read heads.

The preamplifier 830 includes laser driver circuitry 831, writer drivercircuitry 840, and a sensor amplifier 846. The preamplifier 830 includesan amplifier 832 coupled to a laser bias current digital-to-analogconverter (DAC) 834, a laser active current DAC 836, and a lasermodulation DAC 838. The laser bias current DAC 834 sets the bias currentof the laser diode 852. The laser active current DAC 836, controlled bya write enable pin (WRE) 816 of the HDD controller 802, sets theoperating current of the laser diode 852 during write operations. Thelaser modulation DAC 838, controlled by the general-purpose input-outputpin (GPIO) 806 of the HDD controller 802, sets the modulation frequencyof the laser input signal produced by the amplifier 832 and applied tothe laser diode 852.

A clock signal is generated by a modulation clock 804 in the HDDcontroller 802. The frequency of the clock signal determines themodulation frequency of the light generated by the laser diode 852.According to various embodiments, the clock signal generated by themodulation clock 804 has a frequency above the transition frequency,f_(T), of the resistive sensor 854. The clock signal generated by themodulation clock 804 is communicated to the GPIO pin 806. When thesignal on the GPIO pin 806 asserts, the signal causes an increase in thelaser diode current. A register in the preamplifier 830 sets the lasermodulation DAC output level, allowing for modulation amplitudeadjustability. The writer driver circuitry 840 includes an amplifier 842coupled to a write current DAC 844. A write data (Wdata) pin 818 of theHDD controller 802 is coupled to the write amplifier 842 and controlsthe polarity of magnetic current supplied to the write head 856.

The resistive sensor 854 of the HAMR recording head 850 is coupled tothe sensor amplifier 846. The output of the sensor amplifier 846 iscoupled to an input of a digitizer (analog-to-digital converter) 810.The output of the digitizer 810 is coupled to a DMA controller 812,which is coupled to a memory 814. A sampling clock 808 is coupled to thedigitizer 810. The sampling clock 808 is synchronized to the modulationclock 804. For example, sampling of the amplified resistive elementsignal by the digitizer 810 can be performed at twice the modulationfrequency of the light produced by the laser diode 852 (e.g., 2 x themodulation clock signal). By sampling at twice the modulation frequencyof the clock signal produced by the modulation clock 804, the samplingperformed by the digitizer 810 captures alternating resistive sensorsignals with the laser diode on and off. Measurements from the digitizer810 preferably correspond to the average amplitude of the resistivesensor signals. The measurements produced by the digitizer 810 arestored in the memory 814 via the DMA controller 812 for laterpost-processing by the firmware of the HAMR drive.

It is understood that the electrical architecture shown in FIG. 8represents a non-limiting embodiment of a system for laser powermonitoring in a HAMR drive using a resistive sensor and high-frequencylaser light modulation. Other electrical architectures are contemplated.For example, in some embodiments, an HDD controller 802 that lacks ADCscan use external ADCs. Sampling and digitization can be performed in thepreamplifier 830 rather than the HDD controller 802, for example. Lasercurrent modulation can be performed by changing the bias current (viathe laser bias current DAC 834) and active current (via the laser activecurrent DAC 836) supplied to the laser diode 852 via register writes.Also, the amplified resistive sensor signal produced by the sensoramplifier 846 can be processed by a Fourier transform analysis circuitoperating at the laser modulation frequency (e.g., same frequency as themodulation clock signal) rather than by the digitizer 810 shown in FIG.8.

Systems, devices or methods disclosed herein may include one or more ofthe features structures, methods, or combination thereof describedherein. For example, a device or method may be implemented to includeone or more of the features and/or processes above. It is intended thatsuch device or method need not include all of the features and/orprocesses described herein, but may be implemented to include selectedfeatures and/or processes that provide useful structures and/orfunctionality. Various modifications and additions can be made to thedisclosed embodiments discussed above. Accordingly, the scope of thepresent disclosure should not be limited by the particular embodimentsdescribed above, but should be defined only by the claims set forthbelow and equivalents thereof

What is claimed is:
 1. A method, comprising: modulating light generated by a light source situated in, at, or near a slider at or above a predetermined frequency, the slider comprising a resistive sensor; communicating the modulated light from the light source, through the slider, and to an intended focus location of the slider; producing, by the resistive sensor in response to the modulated light, a response due purely to absorption of the electromagnetic radiation by the resistive sensor; and measuring the response of the resistive sensor due purely to the absorbed electromagnetic radiation.
 2. The method of claim 1, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime is due to electromagnetic radiation absorption and conduction of heat; and the response produced by the resistive sensor in the high-frequency regime is due to electromagnetic radiation absorption and not from conduction of heat.
 3. The method of claim 1, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime has a first narrow-band power that decreases with increasing modulation frequency; and the response produced by the resistive sensor in the high-frequency regime has a second narrow-band power that is substantially independent of modulation frequency.
 4. The method of claim 1, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime has a first AC component due to electromagnetic radiation absorption and a second AC component due to heat conduction; and the response produced by the resistive sensor in the high-frequency regime includes the first AC component and is substantially devoid of the second AC component.
 5. The method of claim 1, further comprising determining output optical power of the light source using the measured resistive sensor response.
 6. The method of claim 1, further comprising detecting one or both of a change in spacing and contact between the slider and a magnetic recording medium using the resistive sensor.
 7. The method of claim 1, wherein the modulation frequency of the light is greater than about 500 KHz.
 8. An apparatus, comprising: a light source configured to generate light; a modulator coupled to the light source and configured to modulate the light at or above a predetermined frequency; a slider configured for heat-assisted magnetic recording and to receive the modulated light; a resistive sensor integral to the slider and configured to produce a response due purely to absorption of the electromagnetic radiation by the resistive sensor; and measuring circuitry coupled to the resistive sensor and configured to measure the response of the resistive sensor due purely to absorbed electromagnetic radiation.
 9. The apparatus of claim 8, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime is due to electromagnetic radiation absorption and conduction of heat; and the response produced by the resistive sensor in the high-frequency regime is due electromagnetic radiation absorption and not from the heat conduction.
 10. The apparatus of claim 8, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime has a first narrow-band power that decreases with increasing modulation frequency; and the response produced by the resistive sensor in the high-frequency regime has a second narrow-band power that is substantially independent of modulation frequency.
 11. The apparatus of claim 8, wherein: the predetermined frequency defines a transition frequency between a low-frequency regime and a high-frequency regime; the response produced by the resistive sensor in the low-frequency regime has a first AC component due to electromagnetic radiation absorption and a second AC component due to heat; and the response produced by the resistive sensor in the high-frequency regime includes the first AC component and is substantially devoid of the second AC component.
 12. The apparatus of claim 8, wherein the measuring circuitry is further configured to determine output optical power of the light source using the measured resistive sensor response.
 13. The apparatus of claim 8, further comprising a detector coupled to the resistive sensor, the detector configured to detect one or both of a change in spacing and contact between the slider and a magnetic recording medium using the resistive sensor.
 14. The apparatus of claim 8, wherein the modulation frequency of the light is greater than about 500 KHz.
 15. An apparatus, comprising: a slider configured for heat-assisted magnetic recording; a resistive sensor integral to the slider; a light source configured to generate light; a modulator coupled to the light source and configured to modulate the light at or above a predetermined frequency, the predetermined frequency defining a transition frequency between a low-frequency regime and a high-frequency regime, the low-frequency regime associated with resistive sensor heating due to electromagnetic radiation absorption and conduction of heat, and the high-frequency regime associated with resistive sensor heating due to electromagnetic radiation absorption; and measuring circuitry coupled to the resistive sensor and configured to: measure a response of the resistive sensor in the high-frequency regime; and determine output optical power of the light source using the measured resistive sensor response.
 16. The apparatus of claim 15, wherein: the response produced by the resistive sensor in the low-frequency regime has a first narrow-band power that decreases with increasing modulation frequency; and the response produced by the resistive sensor in the high-frequency regime has a second narrow-band power that is substantially independent of modulation frequency.
 17. The apparatus of claim 15, wherein: the response produced by the resistive sensor in the low-frequency regime has a first AC component due to electromagnetic radiation absorption and a second AC component due to heat; and the response produced by the resistive sensor in the high-frequency regime includes the first AC component and is substantially devoid of the second AC component.
 18. The apparatus of claim 15, wherein the measuring circuitry is configured to measure an average amplitude of the resistive sensor response.
 19. The apparatus of claim 15, further comprising a detector coupled to the resistive sensor, the detector configured to detect one or both of a change in spacing and contact between the slider and a magnetic recording medium using the resistive sensor.
 20. The apparatus of claim 15, wherein the modulation frequency of the light is greater than about 500 KHz. 